1. Field of the Invention
The present invention relates to insulating magnetic metal particles, and a method for manufacturing an insulating magnetic material.
2. Description of the Related Art
In recent years, downsizing and weight saving of electronic communication equipment are intended with rapid increase of communicatory information. As a result, it is desired to reduce the size and the weight of electronic parts to be loaded on such equipment. In the existing portable communication terminals, most information transmission is conducted by means of transmission and reception of radio signals. A frequency band of radio signals which is applied at present is in a high-frequency region of 100 MHz or higher. For this reason, attention is currently focused on such electronic parts and substrates being effective in the high-frequency region. Furthermore, radio signals in a high-frequency region of a GHz band are used in portable mobile communications and satellite communications.
In order to respond to the radio signals in such a high-frequency region as described above, it is required that an energy loss or a transmission loss is small in electronic parts. For example, in an antenna device indispensable for portable communication terminals, the electromagnetic radiation emitted from the antenna cause a transmission loss in the course of transmission. The transmission loss is consumed as a thermal energy in electronic parts and printed circuit boards, whereby it becomes a cause for generating heat in the electronic parts. As a consequence, the radio signal to be transmitted to the outside is cancelled, so that it is necessary to transmit an excessive high-power radio signal, resulting in a setback for effective utilization of electric power.
On the other hand, respective electronic parts come to be downsized with the increase of demands for downsizing and light weight, whereby space saving is intended. In this respect, however, an antenna device is absolutely imperative for assuring a distance from the electronic parts and the printed circuit boards in order to suppress a transmission loss from the reason as mentioned above. Because of this, it is compelled to include an unnecessary space, and as a result, it is difficult to intend realization of space saving.
From such background as that described above, an antenna device using dielectric ceramics is developed, which makes it possible to achieve space saving as a result of downsizing the antenna device. However, since a transmission loss increases due to a dielectric loss in a dielectric material, transmission and reception sensitivity cannot be obtained so that such an antenna device is applied as an auxiliary antenna device in the actual situation. Thus, there is a limitation in electric power saving.
As an antenna device, there is known one which includes an insulating substrate of a high relative magnetic permeability and which performs transmission and reception without transmitting electromagnetic radiation to electronic parts and printed circuit boards in communication equipment by diverting the electromagnetic radiation that reaches the electronic parts and printed circuit boards from the antenna into the insulating substrate. However, a metal or an alloy is used in a usual high relative magnetic permeability material, so that when the frequency of electromagnetic radiation increases, a transmission loss due to eddy currents becomes remarkable, and hence, it becomes difficult to use for an antenna substrate.
Furthermore, it becomes possible to suppress the transmission loss due to eddy currents by an antenna device provided with a magnetic body of an insulating oxide represented by ferrite as an antenna substrate. However, such an antenna device approaches a resonant frequency at a high frequency of several hundreds of hertz, whereby a transmission loss due to resonance becomes remarkable. For this reason, such an insulating high relative magnetic permeability material as that described hereunder is desired as a material of an antenna substrate. Namely, a transmission loss of the insulating high relative magnetic permeability material is suppressed as much as possible, so that it is possible to use the material for electromagnetic radiation of high frequency.
As to an insulating high relative magnetic permeability material, known is a method for manufacturing a thin film nanogranular material of a high relative magnetic permeability by the use of a thin film technique such as a sputtering method. The thin film nanogranular material has a structure obtained by dispersing magnetic metal particles into an insulator in a high density. However, large-scaled facilities are required for practicing the method. Moreover, since a film formation rate is very slow according to the method, it is difficult to thicken the film. In addition, uniform film quality is hardly obtained, so that the method has little practicability in view of a cost and a yield ratio.
Also known is a method for manufacturing a thin film nanogranular material of a high relative magnetic permeability by mixing/dispersing magnetic metal particles with/into an insulating material. However, when a ratio of the magnetic metal particles increases with respect to the insulating material in the method, the magnetic metal particles agglomerate with each other to decrease the dispersibility, so that the magnetic loss increases.
On the other hand, JP-A 2004-281846 (KOKAI) discloses a method for preparing a thick film nanogranular material of a high relative magnetic permeability. In the method, a hardly reducible metal oxide such as SiO2 is admixed with a magnetic metal oxide consisting of at least one of Fe, Co, or the alloys thereof to obtain a layer. The layer is heated in a reducing atmosphere to obtain a sintered body having a powder or polycrystalline structure, while magnetic metal particles are precipitated in the sintered body to obtain the thick film nanogranular material.
JP-A 2004-290730 (KOKAI) discloses a composite particle having a core-shell structure, for example, a composite particle composed of a core made of iron oxide having 0.5 to 10 μm thickness and a shell made of SiO2 having 20 nm to 0.1 μm thickness.
In JP-A 2006-97123 (KOKAI), there is disclosed, for example, a core shell particle obtained by covering a magnetic metal nucleus of 10 μm or less with a multi-layered inorganic material, or a particle obtained by covering further the core shell particle with a resin.
However, the high relative magnetic permeability magnetic material disclosed in the above-described JP-A 2004-281846 (KOKAI) takes a conformation wherein the magnetic metal particles are precipitated in the sintered body having the powder or polycrystalline structure. Therefore, the size of the magnetic metal particles or the distances among the particles are dependent on eventuality, so that the controllability is low and there is little practicability in view of the yield ratio.
In the above-described JP-A 2004-290730 (KOKAI), for the sake of manufacturing a magnetic film from the resulting core-shell composite particles, the shell is molten as a binder to be incorporated into a single body, so that even the core itself is molten, whereby, for example, the spherical core shape thereof deforms to decrease the magnetic property (relative magnetic permeability).
In JP-A 2006-97123 (KOKAI) described above, the outermost layer must be fused for incorporating the resulting core-shell particles into a single body. In the embodiment and examples, the metal particles in the core part exhibits a lower melting point than that of the outermost layer in the case where the outermost layer is an oxide and a nitride. For this reason, the core magnetic metal particles agglomerate with each other, so that the magnetic loss due to eddy currents becomes remarkable. On one hand, agglomeration of the core magnetic metal particles can be prevented in the case where the outermost layer is a resin. However, it is difficult to form a resin layer to be in a thin state. Besides, since a ratio of the core magnetic metal contained in the incorporated magnetic body is small, it becomes difficult to obtain a high relative magnetic permeability. In addition, it is hard to afford magnetism to the resin itself, whereby it becomes difficult to obtain a magnetic coupling among magnetic metal particles in an insulating magnetic material having magnetic particles incorporated into a single body.